This invention relates to fuel elements, and more particularly to fuel elements which are subject to internal pressure buildup during operation.
In many designs of nuclear reactors, the reactor vessel has an inlet and outlet for circulation of a coolant in a heat transfer relationship with a core contained therein that produces heat. The core comprises an array or arrays of fuel assemblies which contain fuel elements. The fuel element is generally a cylindrical metallic sheath sealed at both ends containing nuclear fuel. The nuclear fuel which may be, for example, ceramic fuel pellets of a uranium compound is stacked in the fuel elements. During reactor operation, the nuclear fuel pellets decompose releasing fission products such as fission gas while generating heat in a manner well known in the art. This decomposition of the fuel pellets is sometimes referred to as fuel burnup. The reactor coolant absorbs the heat while circulating through the core thereby cooling the fuel elements of the core and heating the coolant. Of course, the heated coolant may then be used to produce power in a conventional manner.
A common problem associated with these types of fuel elements is high stress levels in the metallic sheath caused by the relatively high external fuel element pressure of the reactor coolant as compared to the internal fuel element pressure. This high pressure difference and resultant high stress levels of the metallic sheath may cause the metallic sheath to rupture thereby releasing fission products into the reactor coolant. Of course, this release of radio-active material into the reactor coolant may cause severe problems and should, therefore, be avoided.
One method of avoiding this problem has been to initially internally pressurize the fuel element with a filling gas. While this initial pressurization reduces the pressure differential across the metallic sheath which thereby reduces the stresses in the sheath at reactor operating conditions, it increases the pressure differential at non-operating conditions. Moreover, as the reactor operates the fuel pellets decompose releasing fission gases while giving off heat. These fission gases which are part of the fission products that must not escape from the fuel elements, mix with the filling gas and increase the internal pressure of the fuel elements so that after a period of reactor operation the internal pressure exceeds the external pressure which results in high stress levels in the metallic sheath. Even where initial internal pressurization is not implemented, the fission products may accumulate to such an extent as to eventually over pressurize the fuel element which again results in high stresses in the metallic sheath. The usual attempted solution to the fission gas build-up problem has been to leave a space for the collection of these fission gases. Merely leaving a space for the fission gases is not totally satisfactory because at least a portion of the fuel element is initially under-pressurized while if a sufficient void is not provided the fuel element may become over-pressurized during operation. In addition, there are several concepts known in the art which attempt to cope with this problem.
In U.S. Pat. No. 3,647,622 -- H. N. Andrews et al., issued Mar. 7, 1972 and assigned to the present assignee, there is disclosed a metallic clad sealed fuel element for a nuclear reactor which may be initially pressurized to resist creep collapse in the early stages of burnup and which has one or more normally sealed plenum chambers which are automatically punctured when predetermined increased pressures are reached during burnup to provide void space for fission gases. In a particular example of the Andrews patent the plenum chambers each comprise an elongated bellows-like structure which when compressed by the increasing pressures within the fuel element cause a wall thereof to be punctured by a pin mounted in the bellows-like structure. In the Andrews concept the internal pressure will increase until a puncturable chamber ruptures at which time the internal pressure will decrease to a certain level until further release of fission gases causes the internal pressure to again increase. As can be seen, the Andrews patent describes a method of accommodating increased fission gases within the fuel element, but in a manner that does not provide a relatively constant internal fuel element pressure.
Another method of accommodating fission gases is described in U.S. Pat. No. 3,647,623 -- M. B. L. Hepps et al., issued Mar. 7, 1972 and assigned to the present assignee. The Hepps patent describes a metallic clad fuel element for nuclear reactors which has a bellows-like member internally supported therein and in direct fluid contact with both the internal and external environment of the fuel element so as to maintain an internal fuel element pressure substantially equal to the external fuel element pressure during reactor operation. In the Hepps patent, a bellows-like member is attached internally to an end of a fuel element. The end of the fuel element to which the bellows-like member is attached has a hole therein which allows the reactor coolant, which may be water, to enter the bellows-like member. The expansion and contraction of the bellows-like member in response to pressure changes of the reactor coolant thereby changes the internal volume of the fuel element which in turn changes the internal fuel element pressure so that the internal and external fuel element pressures are substantially equal. While the Hepps patent does describe the use of a bellows-like member for substantially reducing pressure differentials across the fuel element, it does so by using the bellows-like member as a partial substitute for the metallic sheath. The bellows-like member, therefore, becomes a part of the primary boundary enclosing the fission products in the fuel element. Of course, the bellows-like member being flexible is not as strong as the metallic sheath which means that the bellows-like member becomes the weakest point of the primary fuel element boundary. Since the ultimate purposes of pressure equalization is to prevent rupture of the primary boundary, which is usually the metallic sheath, and thereby to prevent release of fission products, it may not be advisable to utilize a bellows-like member as a part of the primary boundary.
Another attempted solution of the prior art is described in U.S. Pat. No. 3,291,698 to P. Fortescue, issued Dec. 13, 1966. The Fortescue patent suggests that a portion of the fuel element's metallic sheath be made flexible and that a substance which is chemically compatible with the other components, such as a liquid metal, be placed inside the flexible portion of the fuel element. These flexible portions of the metallic sheath are meant to act as bellows-like members to balance the internal fuel element pressure with the external fuel element pressure while the liquid metal supports the flexible portion against collapse. The Fortescue patent also suggests that a gas may be used in conjunction with a liquid metal in such a manner that when the flexible portion of the sheath is deformed inwardly the gas is compressed to the pressure of the liquid metal thereby making the internal pressure uniform over the section containing the gas and liquid. However, it is to be noted that this fuel element is suggested for a rather low pressure environment of a gas cooled reactor and that this use enables the flexible sheath to be relatively thin in order to act as a bellows. If a material having a low modulus of elasticity were used in the area of the flexible sheaths it might only withstand a few cyclic operations in a high pressure environment. Moreover, such a fuel element would be difficult to laterally support within the grid structures now used in high pressure environments of water cooled nuclear reactors.